1. Field of the Invention
The present invention relates to surface wave filter elements used in high frequency circuits of an audio apparatuses, video apparatuses, communication apparatuses or the like.
2. Description of the Related Art
Various surface wave filter elements have been used in circuits of audio apparatuses, video apparatuses, communication apparatuses or the like. A surface wave filter element is basically comprised of a piezoelectric element portion on which elastic surface waves propagate and comb-like IDT electrodes for inputting and outputting signals.
For frequencies in a relatively low range, dielectric ceramics are used for the piezoelectric element portion of the surface wave filter elements. Surface wave filter elements for mobile communication apparatuses using relatively high frequency bands on the order of 1 GHz use single crystal substrate materials such as LiNDO.sub.3, LiTaO.sub.3 and quartz as the piezoelectric element portion.
Meanwhile in general, when the number of users of a frequency band increases to the limit, the frequency band used is moved into a higher band. For example, when the limit for utilization is reached for a frequency band from 800 MHz, to 900 MHz, which is a frequency band used for portable telephones and cellular telephones, the used band is changed to a 1.4 GHz band, a 1.9 GHz band, and a 2.4 GHz band in the order listed. In this case, a surface wave filter element used in these devices is made adaptable to the higher frequencies.
One factor which determines the center frequency of a surface wave filter element is the line spacing and line width of the comb-like IDT electrodes. Therefore in order to raise the center frequency, it is necessary to make the line spacing tighter and the line width smaller with higher accuracy. But it is difficult to make the tight and narrow line spacing and line width . Then it becomes necessary to make a propagation velocity of elastic surface waves fast, which is a value specific to the material used.
However, the conventionally used ZnO, LiNbO.sub.3 or LiTaO.sub.3 provides an elastic surface wave propagation velocity on the order of 3 Km/sec. Therefore for example, it is difficult to form the surface wave filter element whose center frequency is approximately 2 GHz or higher by using photolithography utilizing a stepper apparatus. For surface wave filter elements utilizing such piezoelectric materials adapted to frequencies higher than this, an apparatus utilizing X-rays or electron rays as a light source for photolithography is required, which is not easy to fabricate and, therefore, results in a cost increase.
Then as a method for obtaining higher center frequencies, such a surface wave filter element is known which has a film on which elastic surface waves propagate and makes the film by forming a c-axis-oriented ZnO film on a substrate of glass or sapphire perpendicular to the surface thereof.
Further surface wave filter elements fabricated using a material obtained by forming a diamond polycrystalline film on a substrate made of an inorganic material and by forming a ZnO film thereon have been reported (for example, refer to 1989, ULTRASONICS SYMPOSIUM-351).
However one requirement for the structure of a surface wave filter element to be fulfilled to realize the high frequency characteristics is that the surface irregularity of the film thereon should be sufficiently smaller than the wavelength of elastic surface waves propagating thereon. That is the wave length of a signal propagating through the filter element is determined by the propagation velocity of the surface wave and a pitch of the IDT electrodes and therefore the irregularity of surface causes irregularity of the pitch to affect the precision of the wave length. However a diamond film made up of a polycrystalline substance has irregularity (20-30 .mu.m) on the surface thereof on the order of the crystal grain diameter (20-30 .mu.m) and can not be used for a surface wave filter element as it is. Therefore, it is mirror-finished before use by polishing. However, such a film is difficult to polish because of the following reasons. Although the hardness of the diamond and the boron is similar, namely the diamond is 10 Mose, the boron 9.5 Mose, the amount of abrasion is very different to each other. For example in order to make a rough surface of 10 .mu.m a smooth surface of about 50 nm by using diamond polishing paste, 150 min is necessary in case of the diamond while 5 min is necessary using the crystalline boron material. Therefore the crystalline boron material is very easy to polish as compared to the diamond. Meanwhile the amorphous boron a very smooth surface of 0.1 .mu.m or less .
Meanwhile the boron diamond polishing paste is a best among various polishing materials.
Further the surface smoothness is also required from such view point that the thickness of the ZnO formed on the diamond film is about only 0.1 .mu.m .
As described above, it is not possible to easily obtain a low cost surface wave filter element whose center frequency is approximately 2 GHz or higher using conventional materials.
A surface wave filter element utilizing a diamond film which can have a center frequency higher than 2 GHz is difficult to make smoother on the surface of the film and difficult to fabricate at low cost.